Geochemical model of a magmatic-hydrothermal system at the Lastarria volcano, northern Chile.

Lastarria volcano (25A degrees 10'aEuro parts per thousand S, 68A degrees 31'aEuro parts per thousand W; 5,697 m above sea level), located in the Central Andes Volcanic Zone (northern Chile), is characterized by four distinct fumarolic fields with outlet temperatures ranging between 80A degrees C and 408A degrees C as measured between May 2006-March 2008 and April-June 2009. Fumarolic gasses contain significant concentrations of high temperature gas compounds (i.e., SO(2), HCl, HF, H(2), and CO), and isotopic ratios ((3)He/(4)He, delta(13)C-CO(2), delta(18)O-H(2)O, and delta D-H(2)O) diagnostic of magmatic gas sources. Gas equilibria systematics, in both the H(2)O-H(2)-CO(2)-CO-CH(4) and alkane-alkene C(3) system, suggest that Lastarria fumarolic gasses emanate from a superheated vapor that is later cooled and condensed at relatively shallow depths. This two-stage process inhibits the formation of a continuous aquifer (e.g., horizontal liquid layer) at relatively shallow depth. Recent developments in the magmatic gas system may have enhanced the transfer and release of heat causing shallow aquifer vaporization. The consequent pressure increase and aquifer vaporization likely triggered the inflation events beginning in 2003 at the Lastarria volcano.